In the first of a two-part series, learn seven low-cost tips for improving the energy efficiency of refrigerated warehouses.
By Marcus Wilcox
Refrigerated warehouse are some of the most energy-intensive industrial facilities on the planet. In fact, energy consumption represents the second-highest operating expense in these facilities, trailing only labor costs.
As cold storage companies seek to reduce the cost and environmental impact of their operations, they have two options. They can pursue capital improvement projects, which offer substantial savings but require investments in engineering, design, bidding and project management.
Or they can work to improve the energy efficiency of their existing systems—typically at little or no cost. These adjustments, repairs and fine-tuning may take some time and ongoing maintenance, but the resulting cost savings are invariably worth the effort.
In this two-part series, we explore 13 low-cost tips for improving the energy efficiency of refrigerated warehouses, culled from Cascade Energy’s experience tuning and optimizing the systems at more than 200 cold storage facilities. Part one focuses on refrigeration systems, the heartbeat of a cold storage facility. With numerous components and subsystems, industrial refrigeration systems offer several opportunities for energy efficiency enhancements.
- Evaporators – Evaporators are the “front line” of a refrigeration system. This is where heat is removed from the warehouse, and peak performance is required. Often out of sight and out of mind, evaporators can get dirty and drift out of tune. They also accumulate ice (mostly moisture that has entered through doorways). To maximize energy efficiency, evaporators must be cleaned on a routine basis. Also, the hand expansion valve, hot defrost controls and space temperature probes should be calibrated, monitored and serviced when necessary.
- Compressors – Compressors raise the pressure and temperature of refrigerant, helping move the absorbed heat outside the building. Most modern screw compressors have a microprocessor panel and control sensors on the package. If the control panel and sensors are not calibrated correctly, compressor performance can be negatively impacted. Critical set points include economizers, pressure sensors, variable frequency drive (VFD) settings, current transducers and compression ratio settings. In addition, mechanical repairs on shaft seals, pressure regulators and oil separators may be required.
- Condensers – As the “back door” of a refrigeration system, condensers allow heat to exit the system. If this back door isn’t working properly, system pressure and temperature rise while efficiency and capacity drop. Although condensers, which are typically on the roof, can be difficult to access, cleaning and maintaining their water spray nozzles, pump strainers and belts will greatly improve energy efficiency. Good water treatment is critical to avoid scaling on the tubes and the resulting reduction in performance. Make sure the winterizing heat trace and sump heaters are functioning so the refrigeration system will continue to perform effectively in frigid weather.
- Valves and regulators – Industrial refrigeration systems contain numerous valves and pressure regulators that are spread throughout a facility. These valves and regulators have moving parts that can malfunction or fall out of adjustment. Dirt or impurities can clog mechanisms, while wiring, fuses and control modules can fail. Valves and regulators should be inspected and calibrated on a routine basis, and repaired when necessary. Often, a quick glance at frost patterns will indicate issues that need to be addressed.
- Air and water in the system – Like unwelcome guests, air and water can enter the refrigeration system. Air migrates to the top of the system, negatively impacting the efficiency of the condensers. Purgers are used to remove this air, but they must be monitored to ensure they are functioning properly. Water, on the other hand, dilutes and impacts the boiling point of the system’s ammonia – reducing system efficiency. A dedicated water removal system or a hybrid air/water purger should be installed to maximize performance and efficiency.
- Computer control – A centralized refrigeration control system can manage or monitor all components of a refrigeration system. While these control systems have become much more sophisticated in recent years, they have also become more complex, with myriad control algorithms, set points and energy efficiency features that may be hidden, non-intuitive or downright intimidating. However, fine- tuning these control systems—and using them to take advantage of demand response programs or varying utility rate schedules—can be the greatest source of energy and cost savings. This requires a deep understanding of not only the computer control system, but also how the refrigeration system as a whole consumes energy.
- Variable frequency drives (VFDs) – Standard single-speed induction motors can be found throughout an industrial refrigeration system, driving compressors, fans and pumps. Controlling the speed of these motors and optimizing partial load performance with variable frequency drives (VFDs) can greatly improve system efficiency.
Typically, a central refrigeration control system manages the VFDs, making embedded control algorithms and set points critical. In addition, all VFDs have internal parameters (e.g., carrier frequency, torque curves, minimum and maximum speeds). It is vital that the VFDs are doing what the control system is telling them to do.
Stay tuned for part two in March-April issue of COLD FACTS, which will focus on facility systems, utility rate schedules and other energy efficiency opportunities.
Marcus Wilcox is President of Cascade Energy, a GCCA Service Partner that works with corporations, utilities and industry associations to improve energy planning, engineering and management. For more information, visit www.cascadeenergy.com.